The present invention relates to a method for producing 4-methoxymethyl-2,3,5,6-tetrafluorobenzenemethanol by a selective monomethylation of 2,3,5,6-tetrafluoro-1,4-benzenedimethanol.
Prior, it has been known that certain ester compounds in which their alcohol part is 4-methoxymethyl-2,3,5,6-tetrafluorobenzenemethanol exhibit an excellent insecticidal effect in EP-54360A. A concrete method for producing 4-methoxymethyl-2,3,5,6-tetrafluorobenzenemethanol is known in the scheme below: 
As 1,2,4,5-tetrafluorobenzene is utilized as a starting material and two functional groups of methoxymethyl group and hydroxymethyl group on the benzene ring are constructed separately, the above-mentioned method has many steps. Therefore, the method is not sufficient for economical process in a large scale.
In these situations, a beneficial method for producing 4-methoxymethyl-2,3,5,6-tetrafluorobenzenemethanol is desired to be developed.
Under these circumstances, 2,3,5,6-tetrafluoro-1,4-benzenedimethanol, which is readily available because it has a symmetric structure and can be easily prepared, was selected as a starting material and the method for producing 4-methoxymethyl-2,3,5,6-tetrafluorobenzenemethanol by a monomethylation was developed.
It is generally difficult to producing a monoalkylated compound in high yield by utilizing a compound having two hydroxy groups which have the same reactivity as a starting material. Because a side reaction of producing a dialkylated compound by further alkylating the monoalkylated compound may be accompanied by a production of the monoalkylated compound. 
The present inventors have earnestly studied the condition of selective monomethylation of 2,3,5,6-tetrafluoro-1,4-benzenedimethanol, and as a result have found the method for producing 4-methoxymethyl-2,3,5,6-tetrafluorobenzenemethanol in high yield to complete the present invention. The obtained 4-methoxymethyl-2,3,5,6-tetrafluorobenzenemethanol has a relatively high purity and can be simply purified by crystallization out of solution or by distillation in high recovery. Therefore, the method is suitable for the production in an industrial scale.
Namely, the present invention is a method for producing 4-methoxymethyl-2,3,5,6-tetrafluorobenzenemethanol (hereinafter, referred to as the objective compound) which comprises i) allowing 2,3,5,6-tetrafluoro-1,4-benzenedimethanol (hereinafter, referred to as the starting compound) to react with an inorganic base in water, and then ii) adding dimethyl sulfate and a water-immiscible organic solvent selected from the group consisting of hydrocarbons and ethers to the reaction mixture. Another present invention is a method for producing the objective compound which comprises i) allowing 2,3,5,6-tetrafluoro-1,4-benzenedimethanol to react with an inorganic base in water and said water-immiscible organic solvent, and then ii) adding dimethyl sulfate to the reaction mixture.
The present method has the following two steps. The first step: Reaction of 2,3,5,6-tetrafluoro-1,4-benzenedimethanol with an inorganic base in water and optionally water-immiscible organic solvent selected from the group consisting of hydrocarbons and ethers. The second step: Obtaining the objective compound by adding dimethyl sulfate and optionally said water-immiscible organic solvent to the reaction mixture. In the second step, the reaction should be carried out in the presence of water and a water-immiscible organic solvent selected from the group consisting of hydrocarbons and ethers. Examples of the water-immiscible organic solvent include aromatic hydrocarbons such as toluene and xylene, aliphatic hydrocarbons such as hexane and heptane and ethers such as t-butyl methyl ether.
The first step is considered a step for preparing salt of the starting compound. It is typically, for example, a preparation of salt of the starting compound by adding the starting compound to an aqueous solution of an inorganic base and mixing them, or by dispersing the starting compound in water, adding an inorganic base to it and mixing them.
Examples of the inorganic base used in the first step include alkali metal hydroxides such as sodium hydroxide and potassium hydroxide. The amount of the inorganic base used in the step is one or more mols, preferably 1 to 2 mols based on 1 mol of the starting compound. The amount of water used as a reaction solvent is one or more parts by weight based on one part of the starting compound, and the amount of water is preferably 3 to 5 parts by weight by reason of good solubility of starting compound salt and of volumetric efficiency.
The reaction temperature is preferably in the range of between 15xc2x0 C. and 65xc2x0 C.
One of the preferable conditions is that the mixing in the first step is carried out until most of the used starting compound change to the salt of the starting compound. In that case, the ending point of the first step can be judged at the time for the starting compound to disappear and for the aqueous solution to be clear. The reaction time depends on the reaction temperature and the reaction scale, but is generally 15 minutes to 20 hours.
The solvent used in the first step is water or a mixture of water and the other solvent. When the other solvent is used, the solvent is preferably water-immiscible organic solvent selected from the group consisting of hydrocarbons and ethers. When a hydrocarbon or ether is used in the first step, there is no need to add the hydrocarbon or ether in the second step.
The prepared mixture of the starting compound salt can be utilized for the second step as it is. In other words, the reaction mixture can be utilized for the second step. The salts are exemplified by mono sodium salt or mono potassium salt.
The second step is typically, for example, a preparation of the objective compound by adding dimethyl sulfate and a water-immiscible organic solvent selected from the group consisting of hydrocarbons and ethers to the reaction mixture. When the reaction mixture contains said water-immiscible organic solvent already, the operation of the second step can be an addition of only dimethyl sulfate. The second step may be a reaction of a salt of the starting compound with dimethyl sulfate in water in the presence of said water-immiscible organic solvent.
The amount of dimethyl sulfate utilized in the second step is one or more mols, preferably 1 to 2.5 mols based on 1 mol of the starting compound. It is preferable that the pH value of the aqueous phase is 10 or more, furthermore preferably 13 or more, in which case the second step reaction proceeds well at around room temperature. Though the pH value of the aqueous phase of the reaction mixture may lower depending on the amount of the inorganic base utilized in the first step, an addition of an inorganic base or its aqueous solution can keep the pH value of the water layer in the preferable range that is described above.
The amount of the water-immiscible organic solvent is one or more parts by weight based on one part of the starting compound utilized in the first step. The amount of water used as a reaction solvent is one or more parts by weight based on one part of the starting compound used in the first step, and the water used in the first step is utilized as it is.
The temperature operated in the second step is usually in the range of 0xc2x0 C. to 100xc2x0 C., preferably 15xc2x0 C. to 65xc2x0 C.
In the second step, it is preferable to perform the reaction essentially in the absence of a quarternary ammonium salt in the reaction system. An addition of the quarternary ammonium salt to the present reaction system increase an amount of a dimethylated by-product of 1,4-di(methoxymethyl)-2,3,5,6-tetrafluorobenzene (hereinafter referred as to the by-product compound). The above-mentioned quarternary ammonium salt means a quarternary ammonium salt utilized in a synthetic reaction generally as a phase transfer catalyst, and is exemplified by tetrabutylammonium bromide, tetrabutylammonium chloride, benzyltrimethylammonium chloride, benzyltriethylammonium chloride and cetyltriethylammonium bromide.
The reaction time of the second step also depends on the reaction temperature and the reaction scale, but is generally 15 minutes to 20 hours.
After the reaction is performed, the reaction mixture is allowed to stand, the layer of the water-immiscible organic solvent is separated and the organic layer is concentrated to give the objective compound. Further, the water layer may be extracted with an organic solvent to recover the objective compound for obtaining high yield. When the extraction is carried out, it is preferable that the pH value of the water layer is 3 or less by reason of extraction efficacy. Examples of the organic solvent utilized for the extraction include aromatic hydrocarbons such as toluene and xylene; aliphatic hydrocarbons such as hexane and heptane; ethers such as t-butyl methyl ether; esters such as ethyl acetate and butyl acetate; and mixtures thereof. The layers of the organic solvent obtained by separation and extraction can be combined and concentrated to give the objective compound.
The objective compound can be purified by usual operations such as crystallization out of a solution, distillation and so on. Especially, crystallization out of solution can give the objective compound having a high purity in high recovery, as the present method gives a relatively high purity of the objective compound.
Examples of the solvent utilized for the crystallization include aromatic hydrocarbons such as toluene and xylene; aliphatic hydrocarbons such as hexane and heptane; ethers such as t-butyl methyl ether and tetrahydrofuran; esters such as ethyl acetate and butyl acetate; alcohols such as methanol, ethanol, propanol, 2-propanol, butanol and t-butyl alcohol; organic acids such as acetic acid, trifluoroacetic acid and formic acid; and water; and mixtures thereof. In the crystallization, the by-product compound, which exists in the crude objective compound in small amount, can be removed almost completely.
The starting compound, 2,3,5,6-tetrafluoro-1,4-benzenedimethanol, can be prepared according to the method described in British Patent publication No. 2,127,013A specification.